The modification of carbon molecular sieves (CMS) by prolysis of carbon-containing compounds in order to deposit carbon in the pores of the sieve is well known. Nitrogen has been the carrier gas used in many such processes. For example in U.S. Pat. No. 3,801,513, Munzner, et al., (1974) there is a description of obtaining CMS for oxygen separation by treating coke having volatile components of up to 5% with a carbonaceous substance which splits off carbon at 600.degree.-900.degree. C., thereby narrowing the pores present in the coke. It is stated that the average pore size of the adsorbent must be below 3 angstroms (Van der Waals diameter) to effect oxygen separation from nitrogen. The average pore diameter can be adjusted by changing the intensity of the treatment. Coconut shell coke is a suitable starting material, among others. A preference is stated for a particle size in the range of 0.1 to 20 millimeters and suitable carbonaceous substances which can be used in the treatment include benzene, ethylene, ethane, hexane, cyclohexane, methanol abnd the like. Nitrogen is used in Example 1 both as a carrier gas for benzene and a cooling gas.
Chihara, et al., Proc. Third Pacific Chem. Eng. Congress, Volume 1 (1983) discloses that CMS which is a pelletized granular activated carbon can be treated by thermally decomposing benzene in a fluidized bed of the CMS to deposite carbon thereon and thereby adjust the overall mass transfer coefficients of oxygen and nitrogen in the CMS. Nitrogen is disclosed as a carrier gas for the benzene.
U.S. Pat. No. 4,458,022, Ohsaki, et al., (1984) refers to several prior art processes for narrowing the micropores of active carbon by precipitating soot in the micropores and describes a method said to provide improved selectivity for separating nitrogen from air. The method involves using coconut shell charcoal and coal tar binder, acid washing, adding coal tar and heating to 950.degree.-1000.degree. C. for 10-60 minutes. The coal tar is said to penetrate into the surface of the active carbon and decompose to grow carbon crystallite on the inner surface of the micropore. It is stated that for PSA separation of nitrogen and oxygen, the oxygen adsorption capacity should be more than 5 milliliters (STP) per gram and the selectivity more than 22 to 23. Nitrogen is used as an inert gas in the heating and cooling phases of this treatment.
Surinova, Khim. Tevrd. Top., Moscow (5) 86-90 (1988) describes obtaining carbon molecular sieves for concentration of nitrogen from air by carbonizing gaseous coals using benzene vapor and inert gas. The inert gas is not identified. In other references such as Japanese Patent Application No. Sho 62-176908 (1987) a method for making carbon molecular sieves suitable for separating oxygen and nitrogen is disclosed involving the use of carbon from coconut shells and coal tar or coal tar pitch binder to form particles which are dry distilled at 600.degree.-900.degree. C., washed with mineral acid and water and dried, and then impregnated with hydrocarbon and heat treated for 10-60 minutes at 600.degree.-900.degree. C. in inert gas, for example nitrogen. In this process the inert gas is not used as a carrier or diluent of the modifying hydrocarbon which instead is impregnated into the carbon base material prior to the heat treatment. It is said that this procedure is superior to the use of hydrocarbons, such as benzene, pyrolyzed in the gas phase so that carbon produced adheres to the carbonaceous surface.
A similar but earlier disclosure appears in Japanese Publication No. Sho 49-37036 (1974) which describes making a carbon molecular sieve by condensing or polymerizing a phenol resin or furan resin so that the resin is adsorbed on the carbon adsorbent and thereafter carbonizing the product by heating. Mixtures of the resins can also be used. The resin forming material is dissolved in water, methanol, benzene or creosote oil and the solution is used to impregnate the carbon adsorbent. Carbonizing is then carried out at 400.degree.-1000.degree. C. in an inert gas and a number of suitable inert gases are suggested such as nitrogen, hydrogen, helium, carbon dioxide, carbon monoxide, and sulfur dioxide.
There is nothing in the prior art which suggest that, other than being inert, the nature of the diluent gas which is used with a gaseous carbon-containing compound in a pyrolysis to modify a carbon molecular sieve has any significance. Consequently, it is understandable that the prior work has consistently used nitrogen, the cheapest of diluent gases, for such service.